The Cabinet Office has been ordered to reveal within 35 days the nature of the undertaking Ashcroft made to become domiciled in the UK when he became a peer in 2000. The move follows an appeal spanning three years through the Information Commissioner's Office (ICO) by the Labour MP Gordon Prentice.

Ashcroft made a promise to become a permanent resident of the UK as a condition of his ennoblement in 2000, a year after he was rejected as a member of the Lords by the political honours scrutiny committee, a rejection he believed was partly based on his residency at the time.

But successive Conservative leaders have since refused to reveal whether he has fulfilled that promise and taken up UK residency. The Cabinet Office, which oversees the peerage system, has declined to reveal what his undertaking involved, citing Ashcroft's privacy and clauses in the Freedom of Information Act that exempt the honours system from scrutiny.

Today's ruling by the commissioner, Christopher Graham, overturns that opinion, claiming the public interest in Ashcroft, the House of Lords and the system of awarding peerages outweighs his individual right to privacy, and the need to keep the honours system confidential in this case.

But it goes further, criticising the secrecy about Ashcroft's position. The ruling says: "Since Lord Ashcroft's ennoblement, the question of where he lives has continued to be raised, leading to speculation that Lord Ashcroft has not satisfied the undertaking he gave. Statements by senior politicians concerning Lord Ashcroft's undertaking have been evasive and obfuscatory and have served to compound this speculation.

"Lord Ashcroft could have ended the speculation about his residency by making a public statement to that effect. He has chosen not to do this. He has furthered the speculation by stating that it is a private matter and, as stated on his website, 'If home is where the heart is Belize is my home'.

"In the commissioner's view there is a legitimate interest for the public to know more about Lord Ashcroft's undertaking. This flows from the legitimate public interest in understanding the process by which Lord Ashcroft's peerage was awarded, knowing the details of any conditions placed upon that award and knowing whether Lord Ashcroft has met what appears to have been a condition to his award."

The ruling heaps fresh pressure on Ashcroft and the Conservative party to reveal his tax status as it emerged that Ashcroft has attended a series of important meetings abroad with the shadow foreign secretary, William Hague. Ashcroft has funded and masterminded the battle in key marginal seats across the country. Moves to outlaw tax exiles from both houses of parliament will be debated in the Commons today.

Last June Hague, who was Conservative leader when Ashcroft received his peerage, told BBC2's Newsnight he had "no reason to believe" Ashcroft had not complied with his undertaking, but refused to say whether he had asked him directly. David Cameron, the Conservative leader, has also refused to divulge the information, but attempted to end the row in December last year by promising to bar so-called "non-doms" from parliament.

The information commissioner's ruling also suggests that there is too much secrecy in the system of awarding peerages, saying today's disclosure is important to increase transparency around Ashcroft's appointment as well as the system "in general". The ruling shows how the Cabinet Office refused to reveal the information, citing Ashcroft's privacy, but changed its argument after receiving representations from his lawyer advising it that the act prevented it from revealing information about an individual when the request did not come from the individual. The information commissioner rejected this on the basis that the information related to Ashcroft in his public office.

It concluded by ordering the Cabinet Office to reveal the details of Ashcroft's undertaking within 35 days. It can appeal within 25 days. A spokesman said the Cabinet Office "will respond in due course".

Ashcroft has repeatedly refused to clarify his tax status in Britain and is separately subject to an Electoral Commission investigation into claims that millions of pounds of his donations to the Conservatives, made through his company Bearwood Corporate Services, were in breach of electoral law. The allegation is that the company was not "carrying on business" in Britain and therefore not eligible to donate.

A spokesman for Ashcroft said: "It's not for Lord Ashcroft to comment on this. It is a matter for the Cabinet Office and the information commissioner."

A Conservative party spokesman said: "This is a matter for the Cabinet Office.

"We initially proposed and have now subsequently supported the government's amendment to the constitutional reform bill which will prohibit those that are not, or treated as, resident, ordinarily resident and domiciled in the UK for tax purposes, from sitting in the House of Lords."